Yet another way to arrive at the result (REF ) in Minkowski space is by using the 3D spin-projection operators of [1]}. The latter are defined according to
\(\Pi ^{(\pm )}_{~\alpha }{}^{\beta }=\frac{1}{2}\bigg (\delta _{\alpha }{}^{\beta }\pm \frac{\partial _{\alpha }{}^{\beta }}{\sqrt{\Box }}\bigg )~,\qquad \Pi ^{(\pm n)}_{~\alpha (n)}{}^{\beta (n)}=\Pi ^{(\pm )}_{~(\alpha _1}{}^{\beta _1}\dots \Pi ^{(\pm )}_{~\alpha _n)}{}^{\beta _n}~,\) 
